Phase Transition Enhanced Thermoelectric Performance in Copper Chalcogenides*

Thermoelectric effects are characterized by the Seebeck coefficient or thermopower, which is related to the entropy associated with charge transport. For example, coupling spin entropy with the presence of charge carriers has enabled the enhancement of \textit{zT} in cobalt oxides. We demonstrate that the coupling of a continuous phase transition to carrier transport in Cu$_{\mathrm{2}}$Se over a broad (360-410 K) temperature range results in a dramatic peak in thermopower, an increase in phonon and electron scattering, and a corresponding doubling of \textit{zT} (to 0.7 at 406 K), and a similar but larger increase over a wider temperature range in the \textit{zT} of Cu$_{\mathrm{1.97}}$Ag$_{\mathrm{0.03}}$Se (almost 1.0 at 400K). The use of structural entropy for enhanced thermopower could lead to new engineering approaches for thermoelectric materials with high \textit{zT} and new green applications for thermoelectrics.